1. Field of the Invention
This invention relates to carbon bodies, and to a method for preparing carbon bodies having an improved resistance to oxidation at elevated temperature and a decreased coefficient of friction.
2. Summary of the Art
Carbon oxidizes in air at a measurable rate at approximately 680.degree. F. while graphite reacts at approximately 340.degree. F. The high degree of reactivity of carbon and graphite at elevated temperatures in oxidizing atmospheres is a well-known phenomenon and therefore has limited the use of this otherwise excellent material in many high temperature applications such as in the rocket and missile field. In industrial applications, the use of carbon anodes in aluminum production and graphite electrodes in steel production is well known. However, oxidative attack at operating temperatures of these processes increases the consumption of the carbon bodies by 15 to 30 percent in many cases. It would thus be desirable if the high temperature reactivity could be eliminated or substantially reduced.
It has been known to impregnate carbon bodies with certain metal salts of organophosphates or pyrophosphates and calcine such impregnated articles to provide a composite of carbon and a metal phosphate having increased resistance to oxidation at elevated temperatures. For example see U.S. Pat. No. 3,029,167. The impregnants are synthesized by reacting stoichiometric amounts of a salt of the metal and an acid orthophosphate or pyrophosphate. The process disclosed provides carbon composites having improved resistance to oxidation, but the impregnants utilized are difficult to synthesize consistently in that the acid orthophosphate or pyrophosphate reacts incompletely with the metal salt and leaves large amounts of excess metal salt in the impregnant product of the synthesis. Thus the chemical composition of the impregnant varies significantly and the performance of the carbon bodies prepared therewith is often erratic. Moreover these impregnants have high viscosities and therefore must be diluted with alcohol prior to impregnation. The addition of alcohol as a diluent decreases the concentration of the metal organophosphate or pyrophosphate in the pores of the impregnated carbon body, and, therefore, the desired higher loading of the metal phosphate in the carbon cannot be obtained.
In one attempt to overcome the above problem (in the use of an inert diluent), a molten inorganic phosphate salt was utilized to impregnate the carbon article. See, for example, U.S. Pat. No. 3,342,627. While this process eliminates the diluent, the molten phosphate salts are generally high in viscosity and thus are not efficient as impregnants as desired. Moreover very high temperatures must be utilized to melt the phosphate salt. For example, a temperature within the range of from about 400.degree. to 1500.degree. C. is suggested for effective impregnation. The equipment and operation requirements of this process are obviously substantial.
Another method of preparing composites of carbon and metal phosphates is disclosed in U.S. Pat. No. 3,666,555 wherein a slurry comprising a particulate aluminosilicate refractory material in a phosphoric acid binder is utilized to impregnate the carbon. A slurry, by its two-phase nature, is difficult to distribute uniformly throughout a porous body such as carbon. See also U.S. Pat. No. 4,292,345 wherein a multi-step method of preparing an oxidation-resistant carbon composite is disclosed. This method comprises sequentially impregnating carbon with orthophosphoric acid and an organic compound comprising silicon, titanium or aluminum. The multi-impregnated carbon is then reacted to form a phosphate of silicon, titanium or aluminum.
It is clear that it would be desirable to provide a carbon composite having increased oxidation resistance without going through a multi-step impregnation process, or utilizing molten phosphate salts, inert diluents, or slurries.
Therefore it is one object of this invention to provide a low temperature method for impregnating carbon bodies in a single step with diluent-free solutions of metal metaphosphate precursors.
It is another object of this invention to provide carbon composites having a relatively higher concentration of a metal phosphate uniformly distributed throughout.
It is another object of this invention to provide a process for impregnating carbon utilizing a solution rather than a two-phase slurry or a molten salt to provide a carbon composite having a multi-valent metal phosphate uniformly distributed therethrough.
Additional objects, advantages and features of the invention will become apparent to those skilled in the art from the following description.